Process for the coproduction of a dialkyl oxalate and tertiary butanol

ABSTRACT

A process for the production of a dialkyl carbonate, e.g. dimethyl carbonate, or a dialkyl oxalate, e.g. dimethyl oxalate, from butane, oxygen carbon monoxide and an alcohol is provided in a series of integrated process steps. In the case of a dialkyl oxalate, the product can be hydrogenated further to produce an ether of ethylene glycol.

This is a division of application Ser. No. 741,250, filed June 4, 1985,now U.S. Pat. No. 4,644,078.

The present invention relates to a continuous process for thecoproduction of tertiary butanol and either a dialkyl carbonate or adialkyl oxalate from butane, oxygen, carbon monoxide and methanol in anintegrated series of process steps.

Dialkyl carbonates, of which dimethyl carbonate is an example, areexcellent solvents and have been used in the manufacture of cellulosenitrate and lacquers. They are also used as components in greases,lubricating oils and the preparation of polycarbonate resins bytransesterification. A potentially more important use of dimethylcarbonate is as an additive to liquid hydrocarbon fuels such asgasoline. Recent research has shown that the addition of dimethylcarbonate to gasoline causes an increase in the octane number of thefinished gasoline. Dimethyl carbonate may therefore be an alternative tolead compounds as an `anti-knock` agent. Furthermore since it is burntduring combustion its degradation products are environmentallyacceptable.

Our European patent application No. 0112172 has recently claimed a novelmethod of preparing dihydrocarbyl carbonates, in particularly dimethylcarbonate by the copper catalysed reaction of a primary or secondaryalcohol, carbon monoxide and a dihydrocarbyl peroxide, for exampledi-tertiary butyl peroxide, at elevated temperature and pressure. Duringthis reaction there is co-produced with the carbonate an alcohol derivedfrom the hydrocarbyl group of the dihydrocarbyl peroxide. Thus, forexample, if the dihydrocarbyl peroxide is di-tertiary butyl peroxide thealcohol coproduced is tertiary butanol.

Dialkyl oxalates, of which dimethyl oxalate is an example, also haveindustrial uses. These uses include applications in analysis and thedyestuff industry and include the use of oxalates as a bleaching agentfor straw. A further potential use for dialkyl oxalates is in theproduction of ethylene glycol. Thus it is known that dialkyl oxalatescan be hydrogenated to the dialkyl ethers of ethylene glycol whichthemselves can either be used or converted into ethylene glycol.

Our European patent application No. 0112171 claims a process for makingdihydrocarbyl oxalates, in particular dimethyl oxalate, by the copperand palladium catalysed reaction of a primary or secondary alcohol,carbon monoxide and a dihydrocarbyl peroxide, for example di-tertiarybutyl peroxide, at elevated temperature and pressure. During thisreaction there is coproduced with the oxalate an alcohol derived fromthe hydrocarbyl groups of the dihydrocarbyl peroxide.

Comparison of the two processes described in these two patentapplications show that the major difference between them is the catalystused; thus for a dihydrocarbyl carbonate a copper catalyst is used whilefor a dihydrocarbyl oxalate a copper and palladium catalyst is required.

It is the purpose of the present invention to provide a process for theproduction of either a dialkyl carbonate or dialkyl oxalate, by a seriesof integrated steps from readily available starting materials and inparticular, from butane gas, oxygen, carbon monoxide and an alcohol. Asthe two processes described above only differ in the catalyst used inthe carbonylation stage, the same integrated process can be used forpreparing either a dialkyl carbonate or a dialkyl oxalate by the correctchoice of catalyst.

Accordingly, the present invention comprises a process for thecoproduction of either a dialkyl carbonate or a dialkyl oxalate andtertiary butanol from butane, oxygen, an alcohol and carbon monoxide byan integrated series of steps which process comprises:

(1) in an isomerisation step isomerising the butane feed to a productrich in isobutane,

(2) in an oxidation step, oxidising the isobutane with oxygen to amixture of tertiary butanol and tertiary butyl hydroperoxide,

(3) in a dehydration step, dehydrating the mixture of tertiary butanoland tertiary butyl hydroperoxide to produce di-tertiary butyl peroxide,

(4) in a carbonylation step reacting the di-tertiary butyl peroxide withthe alcohol and carbon monoxide in the presence of either a coppercatalyst or a copper promoted palladium catalyst to produce respectivelyeither a dialkyl carbonate or a dialkyl oxalate and as co-producttertiary butanol,

(5) in one or more separation steps separating the products of step (4)from each other, any unreacted starting materials and the catalyst.

By dialkyl carbonate or dialkyl oxalate is meant a carbonate or oxalateester in which the ester groups are derived from a C₁ -C₆ alkyl alcohol.The alcohol may be either primary or secondary and can be, for examplemethanol, ethanol isopropanol, tertiary and the like. However, apreferred alcohol is methanol and preferred dialkyl carbonates ordialkyl oxalates are dimethyl carbonate or dimethyl oxalate.

Accordingly a preferred embodiment of the present invention comprises aprocess for the coproduction of either (i) dimethyl carbonate ordimethyl oxalate and (ii) tertiary butanol from butane, oxygen, methanoland carbon monoxide by an integrated series of steps which processcomprises.:

(1) in an isomerisation step isomerising the butane feed to a productrich in isobutane,

(2) in an oxidation step, oxidising the isobutane with oxygen to amixture of tertiary butanol and tertiary butyl hydroperoxide,

(3) in a dehydration step, dehydrating the mixture of tertiary butanoland tertiary butyl hydroperoxide to produce di-tertiary butyl peroxide.

(4) in a carbonylation step reacting the di-tertiary butyl peroxide withmethanol and carbon monoxide in the presence of either a copper catalystor a copper promoted palladium catalyst to produce either dimethylcarbonate or dimethyl oxalate and as co-product tertiary butanol,

(5) in one or more separation steps separating the products of step (4)from each other, any unreacted starting materials and the catalyst.

Unit 1, in which step (1) occurs, is fed with the butane feedstock andcauses isomerisation of the feedstock to a product which is mainlyisobutane. The butane feedstock can be pure normal butane or can be amixture of normal and isobutane. There can also be present in thefeedstock significant amounts, for example up to 10% by weight, of C₁-C₃ hydrocarbons. A preferred butane feedstock, however, is typicallyone containing approximately 70% by weight normal butane and 30%isobutane as might be obtained commercially.

The isomerisation step can be carried out byusing, for example, theprocess described in our British Pat. Nos. 953187 and 953189.

The product from step (1) which comprises mainly isobutane together withsmall amounts of n-butane and traces of C₃ hydrocarbons is then fed toUnit 2 in which step (2), the oxidation of isobutane to a mixture oftertiary butanol and tertiary butyl hydroperoxide, is carried out. Theoxidation of isobutane to a mixture of tertiary butanol and tertiarybutyl hydroperoxide has been disclosed in for example U.S. Pat. No.3,987,115 or U.S. Pat. No. 4,404,406 and is a commercially operatedprocess. The products of the oxidation step are a mixture of tertiarybutanol and tertiary butyl hydroperoxide, which are fed to Unit 3, andunreacted normal and isobutane which are recycled to the isomerisationstage. Oxygen or an oxygen containing gas e.g. air can be used.

In Unit 3, the dehydration stage, the mixture of tertiary butanol andtertiary butyl hydroperoxide is dehydrated with concentrated sulphuricacid to form di-tertiary butyl peroxide and water. The products of Unit3, di-tertiary butyl peroxide and water, present as an aqueous solutionof sulphuric acid, are immiscible and can conveniently be separated bydecantation, the di-tertiary butyl peroxide layer being fed to Unit 4for carbonylation and the aqueous layer containing sulphuric acid beingfed to a concentrator Unit 6 prior to recycling to Unit 3. Thedehydration step is typically carried out in the temperature range20°-80° C. preferably 30°-60° C. and at a pressure of up to 150 psig. Atypical dehydration process using sulphuric acid is described in U.S.Pat. No. 2,862,973.

The di-tertiary butyl peroxide produced in Unit 3 is next fed to Unit 4along with the alcohol and carbon monoxide feedstock and the appropriatecatalyst where step (4), the catalysed carbonylation of the alcohol to adialkyl carbonate or a dialkyl oxalate occurs. In addition to thedi-tertiary butyl peroxide, the alcohol, carbon monoxide and catalyst,Unit 4 is also fed with a solvent in which the carbonylation reactionoccurs. The solvent provides a convenient means for adding the catalystto the reactor and for recovering the catalyst in the separation stage.The solvent is thus conveniently one in which the catalyst is soluble.Examples of such solvents are given in European patent application Nos.0112171 and 0112172 which also gives details of the conditions underwhich the unit is preferably operated. It will be appreciated that theoptimum conditions for operating Unit 4 will depend on the dialkylcarbonate or dialkyl oxalate that is being made. Unit 4 may be a singlereactor but, as the carbonylation reaction is highly exothermic,preferably consists of a series of two or more reactors of increasingtemperature in order to moderate the reaction and reduce the amount ofside products formed.

The liquid products of Unit 4, that is the alcohol,dialkyl carbonate ordialkyl oxalate and tertiary butanol, together with the catalyst andsolvent are separated from any unreacted carbon monoxide and fed to Unit5. The unreacted carbon monoxide is recycled to Unit 4 or may be vented.

Unit 5, in which step (5), the separation of products occurs, consistsof two or more distillation columns. In the first column (Unit 5A) thereaction products are separated from the catalyst and its solvent; thecatalyst and its solvent being recycled to Unit 4, optionally through acatalyst reactivation/purification unit (Unit 7). After the initialdistillation the other three products may be separated if desired in oneor more distillation columns (Unit 5B) and any excess alcohol recycledto Unit 4. The dialkyl carbonate or dialkyl oxalate and tertiary butanolcan be recovered pure, or alternatively a mixed dialkyl carbonate ordialkyl oxalate and tertiary butanol stream can be produced.

If a dialkyl oxalate is produced by the above process this can either berecovered or fed to a hydrogenation unit (Unit 8) where it ishydrogenated to ethylene glycol and the alcohol. The dialkyl oxalate canbe hydrogenated using for example a copper/silica catalyst at elevatedtemperature and pressure. Suitable catalysts and conditions aredisclosed in European Pat. No. 0046983.

The tertiary butanol co-product may also be partially recycled to thedehydration Unit 3 for reaction with tertiary butyl hydroperoxide. Thiscan be particularly useful when Unit 2 is operated under conditions suchthat there is produced more tertiary butyl hydroperoxide than tertiarybutanol. In such a case, the recycled tertiary butanol allows the feedto Unit 3 to be adjusted to a molar ratio of tertiary butylhydroperoxide to tertiary butanol of 1:1 thereby satisfying thestoichiometry of the dehydration step.

The preferred embodiment of the invention is essentially the same asdescribed previously except that the alcohol fed to unit 4 is methanoland the dialkyl carbonate or dialkyl oxalate produced is dimethylcarbonate or dimethyl oxalate. The dimethyl oxalate can be hydrogenatedin Unit 8 to produce ethylene glycol and methanol.

The invention described will now be illustrated by reference to thefollowing example.

EXAMPLE

A butane stream, typically consisting of 70% normal butane and 30%isobutane, was fed to Unit 1 and converted to a mixture containinggreater than 98% isobutane. The isobutane rich product was fed to Unit 2along with an oxygen stream the reactor being maintained at atemperature in the range 133°-138° C. and at 36 bars under conditionswhich converted 50% of the isobutane fed into oxygenated products (molarratio of tertiary butanol: tertiary butyl hydroperoxide=1:1). Theunreacted isobutane was separated and recycled.

In the dehydration stage (Unit 3) a mixture comprising 72 g of tertiarybutyl hydroperoxide and 78 g of tertiary butanol was cooled to 5° C. 70%sulphuric acid (130 g) was added to the mixture and the mixturevigorously agitated. During this time, the temperature of the reactionwas maintained below 15° C. After a typical reactor residence time of 10minutes the product mixture was removed from the reaction zone andallowed to warm to 40° C. During this time two phases formed. The upperphase (109 g) was separated by decantation and fed to Unit 4. The lower,aqueous sulphuric acid was fed to the concentrator prior to recycling toUnit 3.

The first carbonylation reactor of Unit 4 was charged with 109 g ofdi-tertiary butyl peroxide, 47.8 g of methanol, 3.7 g of cuprouschloride and 21 g of 2,6-dimethyl pyridine. Carbon monoxide was alsointroduced so as to correspond to a room temperature pressure of 43bars. The first reactor was held at a temperature of 83° C. The reactionso produced was passed through a series of reactors of increasingtemperature at rates which allowed the reaction to be carefullycontrolled. The last reactor was maintained at 125° C. During the totalreaction time 81/2 hours, carbon monoxide was consumed and it wasnecessary at certain points to introduce further carbon monoxide tomaintain pressure. The product mixture leaving the final reactorcontained 69.6 of dimethyl carbonate.

The product mixture leaving Unit 4 was fed to Unit 5A. In Unit 5 themixture was heated to 60° C. under reduced pressure and the volatilecomponents removed. The involatile residue consisting of the coppercatalyst and 2,6-dimethyl pyridine was removed from the bottom of Unit 5and recycled to Unit 4.

The volatile products of Unit 5A were condensed and fed to Unit 5B wherethe remaining components were separated.

We claim:
 1. A process for the coproduction of a dialkyloxalate andtertiary butanol from butane, oxygen, an alcohol and carbon monoxide byan integrated series of steps which process comprises:(1) in anisomerisation step isomerising the butane feed to a product rich inisobutane, (2) in an oxidation step, oxidising the isobutane with oxygento a mixture of tertiary butanol and tertiary butyl hydroperoxide, (3)in a dehydration step, dehydrating the mixture of tertiary butanol andtertiary butyl hydroperoxide to produce di-tertiary butyl peroxide, (4)in a carbonylation step reacting the di-tertiary butyl peroxide with thealcohol and carbon monoxide in the presence of a copper promotedpalladium catalyst to produce a dialkyl oxalate and as co-producttertiary butanol, (5) in one or more separation steps separating theproducts of step (4) from each other, any unreacted starting materialsand the catalyst.
 2. A process as claimed in claim 1 wherein the alcoholis a C₁ -C₆ alky alcohol and the dialkyl oxalate is a C₁ -C₆ dialkyloxalate.
 3. A process as claimed in claim 2 wherein the alcohol isselected from ethanol, isopropanol and tertiary butanol and theappropriate dialkyl oxalate is produced.
 4. A process as claimed inclaim 2 wherein the alcohol is methanol and the dialkyl oxalate isdimethyl oxalate.
 5. A process as claimed in claim 1 wherein a dialkyloxalate is produced in step (4) and the process further comprises step(6) in which the dialkyl oxalate is hydrogenated to the alcohol andethylene glycol.
 6. A process as claimed in claim 5 wherein the dialkyloxalate is dimethyl oxalate and wherein the dimethyl oxalate ishydrogenated to methanol and ethylene glycol.
 7. A process as claimed inclaim 1 wherein the tertiary butanol co-product of step (4) is recycledto step (3).
 8. A process as claimed in claim 1 wherein step (3) iscarried out in the presence of sulphuric acid as a dehydrating agent. 9.A process as claimed in claim 1 wherein the catalyst from step (1) ispurified and reactivated in a catalyst recovery step.
 10. A process asclaimed in claim 1 wherein step (4) is carried out in a series of two ormore reactors of increasing temperature.